The present invention relates to an image transmission apparatus which encodes an image signal and transmits it in real time over a transmission path (digital line).
As this type of image transmission apparatus, there is an image coding apparatus for use in a television (TV) telephone, a TV conference system or the like. An orthogonal transform differential coding apparatus is an example of this image coding apparatus, and this differential coding apparatus divides one field or one frame of TV signals into plural blocks of signals, subjects an image signal for each block to orthogonal transform, such as a cosine transform, acquires not image data after the transform as it is, but the difference between one field or one frame of transformed data and the previous field or frame of transformed data, and encodes the difference signal.
If the motion of an object on a frame is insignificant, the difference signal attained by such a differential coding apparatus becomes nearly zero, thus requiring a very small amount of codes to be transmitted. This is advantageous in signal transmission. If the object moves quickly, however, the amount of codes increases so that the use of the orthogonal transform circuit with a complicated circuit structure does not improve the coding efficiency so much.
Although there are various transmission rates for a digital line for transmitting such image signals, the conventional coding apparatuses are restricted to a specific one of the transmission rates for a target transmission line, or, even if the apparatuses designed to cope with a certain range of transmission rates, the range would be undesirably narrow.
This is because coding apparatuses should satisfy contradictory requirements in accordance with the line's transmission rates. For instance, in a coding apparatus designed for a low transmission rate, the processing (coding) speed can be slow but the coding efficiency (the ratio of the number of bits of a code to the number of bits of an original signal) should be high. For the one designed for a high transmission rate, on the other hand, the coding efficiency can be low, but the processing speed should be high.
Accordingly, the conventional apparatuses have their processing speeds and coding efficiencies determined on the basis of the transmission rates, and cannot therefore be coupled to those lines with different transmission rates. For instance, if a low transmission rate coding apparatus is coupled to a high transmission rate line, the coding speed cannot meet the line's transmission rate, which renders the apparatus inoperable. On the other hand, if a high transmission rate coding apparatus is coupled to a low transmission rate line, the image is significantly deteriorated due to the low coding efficiency.
Therefore, when a network is constructed by lines whose transmission rates vary between 64 Kbps and 1.5 Mbps, at least two coders/decoders for use for different transmission rates need to be at each communication site as shown is FIG. 1, increasing the installation cost and the installation space required.
To construct an image communication network which is studded with different types of coders/decoders as shown in FIG. 2, it is necessary to confirm the type of the coder/decoder at a destination prior to the image communication and this would complicate the coupling operation.
Further, matching between the coding speed and the decoding speed is important in constructing the image communication network. If the coding speed is higher than the decoding speed, a buffer memory, which is coupled between the coder/decoder assembly and the associated line to match the coding/decoding speed with the transmission rate, would overflow. On the contrary, with the decoding speed higher than the coding speed, the buffer memory would underflow. When such a phenomenon occurs, signals sequentially received would be interrupted and the signal transmission would be synchronous. As a result, information is likely to be lost or wrong decoding may be executed until the next synchronization time. In addition, with the use of a differential coding system, once wrong decoding is done, wrong signals will remain stored in the buffer memory unless the data in the buffer memory which is associated with the wrong decoding is rewritten by movement of an object.
Of those image transmission apparatuses which sequentially encode an image signal itself, not a so called difference signal that does not deteriorate an image quality so much even upon occurrence of loss of information, there is a type which allows for overflow and underflow of the buffer memory. In the TV conference system, however, the overflow and underflow of the buffer memory cannot be allowed for.
Japanese Patent Disclosure (Kokai) No. 60-154752 discloses a coding/decoding apparatus for television camera signals. According to this apparatus, when there is a small amount of transmission information (codes) on the sender side, insignificant information (dummy) is affixed to it and then sent so that the amount of data stored in the buffer memory on the receiver side becomes constant. This can prevent the occurrence of the possible overflow and underflow of the buffer memory on the receiver side.
However, affixing the dummy information to the codes prior to transmission inevitably results in a waste in information transmission and thus leads to inefficient information transmission.
In a differential coding (such as DPCM) apparatus, when some kind of error occurs in a transmitted code due to noise etc., this error would affect the decoding of the next difference signal (code) and errors would be accumulated in the decoder.
To prevent this phenomenon, conventionally error detection is executed on the receiver side, and upon detection of an error, a re-send request is sent to the sender. Then, the original PCM signal, not the DPCM signal, is sent from the sender to rewrite the content of the buffer memory on the receiver side.
However, this type of coding apparatus requires a device to detect the error the receiver side, and requires, on the sender side, a device to receive the re-send request and re-send the PCM signal associated with the transmission error and a buffer memory for temporary storage of the PCM signal to be resent.
Further, when a burst error occurs, the re-send request signal and the resent PCM signal alternately appear on the transmission line, thus reducing the capacitance of the transmission line assigned for transmission of the code data (DPCM signal).
In particular, since the orthogonal transform differential coding system involves block-by-block processes, even if an error occurs only in one of parameters of the converted data, it is necessary to resend the PCM signal associated with all the parameters of the block including the parameter in error (e.g., 8.times.8=64 parameters in the case of a two-dimensional 8th-order orthogonal transform). This system is significantly inefficient. Even if it is sufficient to resend only the PCM signal associated with the parameter in error, it is also necessary to send, at the same time, a signal indicating the location of the error parameter in its own block. This is a very inefficient system.
The difference coding system is used particularly in the case of a low transmission rate. Codes to be transmitted are often irregularly generated, and it is necessary to affix addresses to codes on the sender side, and align the irregularly-generated codes with data in the receiver's buffer memory and add them to the data using these addresses. If, in this case, an error occurs during transmission (i.e., transmission error) and any address is in error as a consequence, the proper address alignment cannot be done on the receiver side, causing the improper decoding. As incorrect data is accumulated in the receiver's buffer memory, adverse influence of the transmission error continues and significantly increases the deterioration of a reproduced image unless the transmitted codes are refreshed.
There have been methods known which use an error detection code or an error correction code in order to prevent the adverse influence of the transmission error. Because of the use of redundant codes for error detection and error correction, however, the codes become lengthy and the apparatus becomes complicated. In this respect, therefore, the methods for using such error detection and correction codes cannot be said to be effective in performing image transmission with high coding efficiency.